This invention relates to phosphate laser glasses, typically neodymium-doped, having high fracture toughness, good cross section for stimulated emission, and, especially, high thermal conductivity and low thermal expansion, inter alia, in comparison to prior art and commercially available phosphate laser glasses.
The term "laser" refers to the amplification of light by the stimulated emission of radiation. In a laser, a suitable active material, which can be a glass suitably doped with an active atomic species such as neodymium, is placed in a cavity resonator formed by two reflecting, or at least partially reflecting, walls.
Solid state lasers used for the generation of high levels of average power require that the active material possess a large value (e.g., &gt;1) of the thermal-mechanical figure of merit, FOM, given by: ##EQU1## where S is the fracture strength; K, the thermal conductivity; v, Poisson's ratio; E, Young's modulus; and .alpha., the thermal expansion coefficient of the material. The fracture strength is not totally an intrinsic property of the material but also depends on the physical condition of the surface of the material as well as the material's fracture toughness.
The importance of the thermal-mechanical figure of merit is clear by considering the thermal condition of a solid state laser material used in a high average power application. During operation, the laser material is exposed to intense levels of pump radiation, a fraction of which is converted by the material into laser emission. A portion of this pump radiation is absorbed by the active material itself resulting in an increase of temperature which rapidly is manifested as a drop in laser efficiency, in some cases serious enough to terminate laser action. As a consequence, it is necessary to cool the laser material by passing a liquid or gas over its surfaces; which, in turn, results in a thermal gradient as the internal temperature of the material rises higher than the temperature of its surfaces. The thermal gradient is accompanied by a stress gradient through the laser material which can be high enough to cause fracture of the active material. The thermal mechanical figure of merit is proportional to the maximum gradient that the material can tolerate without fracture.
To optimize a glass for high average power application it is thus necessary to maximize thermal conductivity and minimize the coefficient of thermal expansion, Poisson's ratio, and Young's modulus. An additional benefit of high thermal conductivity is its direct impact on lowering the internal temperature of the laser material. The effect of minimizing this temperature, for a particular level of pumping, is found both in the reduction of the stress gradient within the material and in a drop in the thermal population of the lower lasing level. This reduction in population of the lower lasing level increases the operating efficiency of the laser.
It is important in the field of high average power lasers that the active material also be characterized by high cross section for stimulated emission and long fluorescence lifetime of the excited state involved in the laser transition. Solid state laser materials are also more favorable for application in high average power laser systems if the active material can be produced in large sizes with high optical quality, high optical homogeneity, and in a form free of all inclusions which absorb radiation. The latter could develop into damage sites within the material when the laser is operated at high power levels, leading ultimately to the active element being rendered useless as a laser material. It is further desirable that the glass materials be chemically strengthenable. Of course, it is always necessary that the glass have good manufacturability properties, e.g., devitrification stability.
It is known that phosphate laser glasses have a low threshold value for the laser effect, and phosphate compositions have been commercially available for some time as materials for use in laser systems in large sizes with excellent optical quality and optical homogeneity. The quality of prior-art phosphate laser glasses recently has been extended by the introduction of new manufacturing technology capable of producing these compositions as glasses with levels of optical quality as good as that of previous glasses but which are now free of all absorbing inclusions which could develop into damage sites within the glass.
Nevertheless, a need has remained to further the development of phosphate compositions, e.g., improve even more the already-excellent thermal-mechanical properties of available phosphate laser glasses, thus making available new compositions which are more attractive for use in high average power laser systems and/or which increase the maximum tolerable power levels, while retaining the current state-of-the-art qualities which make doped phosphate glasses so useful as laser media.
Prior art phosphate glasses contain a wide variety of components including, for example, Al.sub.2 O.sub.3, SiO.sub.2, alkali metal oxides (Na.sub.2 O, K.sub.2 O, Li.sub.2 O, especially), alkaline earth metal oxides, etc., in addition to the base component P.sub.2 O.sub.5. The prior art glasses having the best combination of the important thermal properties of thermal conductivity and coefficient of thermal expansion have typically been those containing necessary amounts of SiO.sub.2. See, e.g., DE 3435133, JP 51-107312 and DE 3609247. These glasses typically have relatively low alumina contents.
Other phosphate laser glasses place no special emphasis on SiO.sub.2 or even lack it entirely, e.g., U.S. Pat. Nos. 4,248,732, 4,075,120, 4,239,645, 4,022,707, 4,470,922, JP 51-59911, DE 2924684, and DE 3340968, etc.
Many other laser phosphate publications exist describing a wide variety of glasses such as JP Nos. 49-114,615(4), 60-191,029(3), 51-107,311, 50-3,411, 51-30,812, SU-355,916, U.S. Pat. Nos. 4,333,848, 3,846,142. In these patents, no particular emphasis is placed on alkali metal oxides. Further patents equate all the alkali metals, e.g., U.S. Pat. Nos. 4,248,732, 4,075,120, 4,120,814, 3,979,322, 4,225,459, 3,580,859 and 4,470,922. Others generically imply that lithium oxide, for example, is less preferred than the other alkali metal oxides. Such patents include U.S. Pat. Nos. 4,022,707, 4,076,541, 4,661,284 and 4,333,848. JP 54-38,311 indicates a preference for lithium but in phosphate glasses containing components such as CuO and V.sub.2 O.sub.5.